Brown widow spider rapidly spreading in California

Thu, 2011-04-28 07:59

There are several reasons why the agricultural community should be concerned about potential northward migration of brown widow spiders.

"The brown widow is spreading like wildfire," said UC Riverside urban entomologist Rick Vetter. "It's a very prolific pest. People find them by the hundreds in places where they haven't seen spiders before."

A UC Riverside scientist is asking for the public's help to track the distribution of brown widow spiders in California. Brown widows' range expanded rapidly in Southern California since their introduction in 2003, he said, and they may move northward this summer into Central California.

"The brown widow is spreading like wildfire," said UC Riverside urban entomologist Rick Vetter. "It's a very prolific pest. People find them by the hundreds in places where they haven't seen spiders before."

Another widow species is the black widow, which is native to California. When the weather warms up, it's not uncommon to find the glossy black spiders with their tell-tale red hourglass abdomen mark in wood piles, under flower pots and in webs strung across undisturbed niches low to the ground.

Brown widow spiders are native to Africa and are established in tropical environments throughout the world. They have been found in Florida for many decades, but only recently expanded their range from Texas through South Carolina, and into southern California. As of 2009, the spider was established in Los Angeles, Orange and San Diego counties, and in 2010 it made its way to Riverside, San Bernardino, Ventura and Santa Barbara counties. There have been a few finds in areas further north.

"I've gotten three females from Sacramento and three females from Washington (state)," Vetter said. "I've gotten no other spiders from those areas, so I don't know if they will be another infestation area or not."

The brown widow poses less of a health threat than black widows, but Vetter said there are several reasons why the agricultural community should be concerned about their potential northward migration. Currently little is known about brown widow spider biological control. While black widows prefer low hangouts, it is not yet known whether brown widows will adjust to higher posts in California. If the spiders take up residence in fruit orchards, for example, they could pose a problem for farmworkers.

"Pickers and harvesters won't want to have these spiders falling down on them," Vetter said.

Brown widows could also potentially congregate in agricultural shipping containers or packaging.

Brown widow venom

Drop for drop, brown widow spider venom is more toxic than the venom of their black cousins, but other characteristics make them less dangerous, according to Vetter. Brown widows are less likely to bite. Instead, they curl up, drop and play dead when disturbed. They do not defend their egg sacs. When brown widows do bite, they release less venom than black widows.

However, brown widows will have the advantage of numbers. Black widows are known for their anti-social tendencies, usually occupying space alone. Brown widows have no such fondness for solitude.

"Where you might find six or seven black widows in a backyard, now you find 100 brown widows," Vetter said.

Vetter is asking the public to assist in his brown widow spider research by carefully following instructions for collecting and sending brown widow spider specimens to the university. Potential spider collectors should study the photos on his website at http://cisr.ucr.edu/brown_widow_spider.html to learn the characteristics of brown widows.

Because the spider is already established in Southern California, Vetter does not need specimens from San Diego, Orange, Los Angeles and the Riverside and San Bernardino-Redlands area. More specimens are welcome from Ventura, Santa Barbara, from Riverside and San Bernardino counties outside of the urban cities in the western part of the counties and from all the rest of California.

Immature black widow spiders are often confused for brown widows because they look alike: mottled tan, brown and gray body with an orange or yellow hourglass on the abdomen. Vetter said the easiest way to distinguish the spiders is their egg sacs. Black widows' egg sacs are smooth, teardrop shaped and a little yellowish; the brown widows' egg sac has spiky protrusions that make it look like a large pollen ball.

"People have sent in a lot of different things already thinking they are brown widow spiders and they've been wrong," Vetter said.

Spiders may be shipped dead or alive. Good quality digital images may also be submitted. For submission instructions and the mail and email address, see Vetter's brown widow spider research page at http://cisr.ucr.edu/brown_widow_research.html.